The technique of using an inductor is usually used in a voltage-controlled oscillator (VCO) including an inductor-capacitor resonator (LC Tank). When the semiconductor manufacturing process causes variations in, for example, the width, length, or oxide layer thinness of a transistor gate with the technology progress, a variable inductor can help to compensate for the variations.
Changing the inductance of an inductor can expand the tuning range of a VCO. In particular, the oscillation frequency of the VCO may require a millimeter wave design, and in this situation, the oscillation frequency may be larger than 30 GHz. In the past, the person skilled in the art tuned a metal oxide semiconductor capacitor (MOSCAP) to adapt to different wavelengths. For a millimeter wave design with a frequency larger than 30 GHz, the MOSCAP technique almost reaches the physical limit. The main reason is that with variations of the manufacturing process, such as the current 20 nm process, the line width decreases and the thickness of the metal layer in the lower layer also decreases. In addition, the higher the frequency is, the smaller the capacitance and inductance that are required. In the semiconductor manufacturing process, MOSCAPs are always formed in the lower metal layer so that resistance experienced by the MOSCAPs will increase quickly. Take the simplest RC series circuit for example, the quality factor, or Q factor, can be represented by 1/(ωRC) wherein ω is angular frequency, R is resistance, and C is capacitance. In this situation, the effect of the capacitance variation on the tuning range will decrease, and, no matter how it is tuned, the Q factor cannot be effectively increased because the resistance is too large. Therefore, one may think about developing a variable inductor. For variable inductors, when inserting a switch between two inductors to serially connect the inductors, the equivalent inductance can be tuned by turning the switch on and off. However, the drawback of this method is that the resistance and inductance of the switch will be included in the series circuit and the overall performance will not approach the original inductor.
More details of the relevant prior art can be acquired from U.S. Pat. No. 7,460,001 and US Patent Application No. 20120223796.